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The previous approach desugared the ast.SelectorExpr
to make implicit field selections explicit.  But:
1) it was clunky since it required allocating temporary
   syntax trees.
2) it was not thread-safe since it required poking
   types into the shared type map for the new ASTs.
3) the desugared syntax had no place to represent the
   package lexically enclosing each implicit field
   selection, so it was as if they all occurred in the
   same package as the explicit field selection.
   This meant unexported field names changed meaning.

This CL does what I should have done all along: just
generate the SSA instructions directly from the original
AST and the promoted field information.

Also:
- add logStack util for paired start/end log messages.
  Useful for debugging crashes.

R=gri
CC=golang-dev
https://golang.org/cl/7395052

R=iant, gri, iant, rogpeppe
CC=golang-dev
https://golang.org/cl/7196053

This CL includes the implementation of Literal, all the
Value.String and Instruction.String methods, the sanity
checker, and other misc utilities.

R=gri, iant, iant
CC=golang-dev
https://golang.org/cl/7199052